Animal life is known to exist on many green roofs across Switzerland, including bees, spiders, butterflies, beetles, and even some ground nesting birds, but no comprehensive study yet has explored grasshopper populations. Listings of green roof benefits often include storm water retention, protection of the waterproofing from UV rays, improved thermal performance of the building, and added aesthetic value to the built environment. However, truly ecological green roofs also offer wildlife the resources needed for feeding, for rest and perching, for mating, and for nesting. In effect, such a living roof must serve as habitat if it is to support wildlife.

Grasshoppers can exist on roofs provided their habitat needs are met. With regards to food, grasshoppers are herbivores with preference for grasses and sometimes sedges. Since grasshoppers are themselves food sources for birds and spiders, they need places to hide (especially as they are so exposed on rooftops). Given their trophic placement on the food chain, the presence of grasshoppers could be used as an indicator to the functionality of a green roof as an ecosystem. This study characterizes the existence of grasshoppers on green roofs of varying age, substrate, and vegetation across northern Switzerland.

Tettigonia viridissima (Wollishofen).

Methodology

This study aims to understand and document the occurrence of self-sustaining grasshopper populations on green roofs. In a brief two-month period (July-Sept 2012), twenty-six (26) green roofs were visited in the Basel and Zurich regions of Switzerland. In addition to grasshoppers species and their approximate densities, substrate and vegetation type (i.e. herbs, grasses, Sedum) were noted. This report presents preliminary results, exploring correlations between species number and surrounding influences.

The grasshoppers were collected by netting and examined on the spot. If the species was recognized from previous encounters, the grasshopper was released. In the case of any questionable identifications, it was photographed using a Nikon D3100 with a standard 55mm lens. If the species was not recognizable, a collection bottle was prepared with a scrap of paper towel soaked in two drops of ethyl acetate. Those grasshoppers were put on pins for further examination.

Results

Of the 26 roofs sampled in Zurich and Basel regions, a total of 11 species were found (Table 1). Of these, five are on the Red List for Basel region. The Red List classification series (3 = vulnerable, 2 = endangered, 1 = critically endangered) referred to here is specific for the Basel region, where a large majority of the roofs are located. The analysis on Orthoptera was done in 2000 by Armin Coray, retired professor at the Zurich University of Applied Sciences (ZHAW).

Table 1: Eleven grasshopper species were observed on 26 green roofs over two months (summer 2012)

For complete roof details, see Appendix 1.

In terms of red-listed species, Platycleis albopunctata (3 on Red List) was found on the BVB green roof. This species tends to prefer higher grasses. It was found in a similar habitat at Muttenz, but it may have just been passing through rather than living out its lifecycle.

Microclimates and varied habitats on the BVB Tram Depot.

A substantial population was also found on Stücki Shopping Centre in Basel, but almost exclusively on large rocks in sparse tall herbs, which is unusual habitat for this species. There is no apparent explanation for this phenomenon. It may be interesting to study this further. Another exceptionally rare species, Aiolopus thalassinus, had not been documented in Basel before (and so is not even on the Red List!). This absence and rarity is likely because this species prefers habitats that have been systematically removed from the region (floodplains bordering on dry and wet). It is listed as endangered on the national Red List for Switzerland. According to Armin Coray:

“There are, however, records [for this species] from nearby areas behind the Swiss border. I could … find a small population in 1988, at a gravel-pit south of Huningue (France), only about one and a half kilometer from the Swiss border. But since 1990 or 1991, this habitat does no longer exist (it has been turned into a parking lot). There is a more recent observation made by Robert Portmann, who found the species on 18th Sept. 2010 at a gravel-pit in Wyhlen (Germany).”

Aiolopus thalassinus (Stücki).

Aiolopus thalassinus was found on Stücki, and may have been an accidental landing that was caught by chance. However, it did look as if there were several of the same species in the particular area this specimen was caught, which could serve as evidence, however inconclusive, of its persistence on rooftops.

The most commonly found species was Chorthippus biguttulus. This is not surprising as this is a very common species in central and northern Europe. It is interesting to note, however, that the buildings of Moos Water Filtration Plant at Wollishofen, Zurich were dominated by another species, Chorthippus dorsatus, whereas C. biguttulus was not found on these roofs at all. The roofs at Wollishofen varied from the others studied in that they are much older, nearing one hundred years, and they are larger and lower to the ground. The vegetation on the Wollishofen roofs is quite dense and includes tall grass. It is disturbed only once annually when the grass is cut, and then a 3 meter strip is left to provide a safe haven for invertebrates. However, many of the grasshoppers fly off at the time of the mowing, which usually happens towards the end of the summer and thus after most of the grasshoppers have mated and deposited their eggs. C. dorsatus was not found on any of the other roofs in this study.

Varied habitats on a large area, Stücki.

In terms of the roofs sampled, the Stücki rooftop overall was the most interesting. This roof had eight different constructed habitats, from bare gravel to dense grasses, and six species of grasshoppers were found there, the most found on any roof visited. Caliptamus italicus, which is uncommon in Switzerland and especially in Basel, was found here. This species was located exclusively in high herbs on a gravel substrate with some compost. Some other red listed species, Sphingonotus caerulans (2 on Red List) and Oedipoda carulescens (3 on Red List) were found in the same habitat of gravel near a pile of dead wood. These species are known for their preference for heat and thus predilection for rocky habitats.

Sphingonotus caerulens (Stücki).

The Stücki Shopping Mall rooftop is unique amongst the other roofs studied because it accommodates the most species and is composed of so many habitat types. The scatter plot in Figure 1 presents a correlation comparing the number of species with the number of habitats on this roof. The habitats were classified into nine different types: 1) dense sedum mat, 2) sparse sedum mat, 3) dense grasses, 4) sparse grasses, 5) small gravel, 6) gravel with herbs, 7) large gravel, 8) sedum and herb mix, and 9) dead wood. The green numbers beside the x’s on Figure 1 indicate the number of matching correlations (i.e., 8 instances of 2 habitats on the roof correlating to 1 species found).

The relationship between grasshopper species diversity and the diversity of substrate and vegetation was investigated using a Pearson product-moment correlation. There was a strong, positive correlation between the two variables, r = .54, n = 26, p< .005, with greater grasshopper species diversity associated with more diverse substrates. This is good news considering the roofs varied in height, size, and age. In this case, it seems that more diverse the substrate, and therefore the vegetation, the more diverse the fauna. So, in designing for biodiverse green roofs, diversity in the depth and material used, despite of the size or age of the roof, will encourage the colonization of more species of grasshoppers.

The dendrogram in Figure 2 shows the relationship amongst roofs in accordance with the species found. For instance, UBS and Exhibition Hall had very similar species composition, while Moos Wollishofen and Stücki are quite different. When these results are compared with the vegetative composition of the roofs, we see a similar correlation, as both UBS and Exhibition hall roofs are basically Sedum monocultures while Wollishofen has one consistent grass covering and Stücki supports a variety of habitats.

Discussion

The data collected at later dates (August) may have embedded inaccuracies owing to the nature of the grasshopper life cycle: it’s possible that more species were collected for the roofs sampled near the end of summer. Though every species has its own unique life cycle, which occurs over 12 months’ time, generally after reaching maturity adults will only survive 4 to 7 weeks before dying. Depending on the climate and the species, this tends to happen throughout August and into September. Furthermore, the species inhabiting green roofs are likely to be lowland species incoming from urban surroundings rather than alpine species that may have a resistance to cold and wind. This matter of timing may explain the low numbers of grasshoppers found on UBS, Rosetti, Kinderspital, Zurichsee, and Novartis. This also calls into question other roofs visited near the end of August (although the weather only got cold and turbulent towards the final days of that month).

Solar panels create microclimate and hiding spots; Kinderspital.

This potential inaccuracy suggests that further research must be done in order for any conclusive results on green roof inhabitance by grasshoppers. For instance, each roof should be visited at least twice, once in the early summer months, and once in the later months. This way, not only could species that were simply “passing through” be ruled out, but species migrations and changes in the species composition of the roofs could also be observed.

Some observations were made of potential “passing through” culprits. For example, on the Muttenz Gym Hall, only three grasshoppers were found – two individuals of Platyceis bipunctata and one of Pheneroptera nana. Also on the Geography Institute, only one individual of Chorthippus biguttulus was found. Without returning to the roofs and investigating further, it is difficult to say whether or not these were regular visitors or not. However, such low numbers seem to indicate accidental landings.

Platyceis bipunctata (Muttenz Gym Hall).

In terms of next steps, a sample size greater than 26 roofs would lead towards a more comprehensive study. In such a case, a more thorough investigation of the conditions for grasshoppers on Swiss green roofs could include estimates of species and individuals on various roofs, review the presence of juveniles (to ensure that the animals are not simply visitors), and analyze for potential source populations, the sizes and ages of the roofs and the plant species composition. In addition, data collection across a longer time scale would benefit the quality and potential of the data. Lastly, sampling the wide range of climates in Switzerland, and thereby including greater variation in habitat potential and population dynamics, could further expand our understanding of the needs and potential for self-sustaining grasshopper populations on green roofs.

Conclusion

It was interesting to observe such a diversity of grasshoppers on green roofs, with everything from the common Chorthippus biguttulus to the new find for Basel, Aiolopus thalassinus, but these data are hardly scratching the surface. However preliminary they may be, the results from this study are encouraging. At least one species was found on the majority of the roofs visited, which suggests that grasshoppers are able to colonize quickly, and in diverse habitats. Since the weather may have affected the low species count towards the end of August, it would be interesting to see whether earlier data collection (e.g. starting in June) would yield different results.

If given more time, will more grasshoppers find this habitat and claim it as a suitable place to breed? To investigate such a question, an estimation of population sizes would be required. In order to rule out grasshoppers that may be “passing through,” one could note if nymphs, who are not able to fly, are present on the roofs.

The strong correlation between habitat diversity and species diversity is fairly obvious and what one would expect. For any environment, the more diverse the area, the more species it will foster. Even so, including more roofs to the dataset would give a more precise measurement of populations and help to determine whether they are static or dynamic.

The prospect of another invertebrate group finding habitat on green roofs is exciting. Given that grasshoppers are more likely to be found in meadow habitats with grasses and herbs, rather than Sedum mats, further substantiates the basis of green roof designs for biodiversity: by mirroring existing ecosystems on rooftops, we can create attractive habitats for a diversity of organisms, certainly by contrast to shallow and xeric green roof systems. This work, and the associated research conducted by the ZHAW, is facilitating a greater understanding of how the built environment can support the needs of both humans and other species, and it seems that we are moving in the right direction.

The author at the Moos Lake Water Filtration Facility.

A special thanks to Dr. Patrik Wiedemeier, who taught me how to identify grasshoppers and verified each identification I made. Also thank you to Manuel Speck who showed me the art of catching the little critters, helping me make appointments for roof visits, and visiting some roofs with me. Thank you to Christine Thuring for her edits. Thanks especially to Dr. Stephan Brenneisen who introduced me to new friends and colleagues and gave me the direction and opportunity to complete this work.

Julie Ann Razryadov, Founder of Downtown Binghamton Greenroof Initiative in Binghamton, New York, has conducted independent research on green roof technology, economics, and sustainability. With a BA in Environmental Studies, Julie has dipped her hands in climate action planning, recycling campaigns, clean energy bike tours, organic gardening, and a variety of food justice outreach programs. Her interest in green roofs and botany has brought her around the world and back again, through Switzerland, New Zealand, and Malaysia. She dreams of wide spread native green roof plantings across the United States.

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